This project represents continuation of a long-term program aimed at understanding developmental implications of those complex cell interactions by which the host marshals an immune response to tumors, allografts of virus infected cells. Six approaches will be pursued: 1) Analysis of a model for MLR involving triggering of specific recognition responses in immunocompetent thymus cells toward allogeneic targets, by B-helper cell components; 2) defining the mechanism whereby T-cell activation nonspecifically triggers greatly augmented B-cell proliferation and antibody production; 3) further dissection of the complex interactions between the host lymphoreticular system and tumors, including definition of cell subclasses involved in cell interactions; 4) exploration of the possible relationship between tumor specific transplantation rejection responses, and histocompatibility determining structures governed by the MHC locus of mice; 5) exploration of the role of MuLV virus expression by tumor cells in host rejection responses; and 6) defining possible roles of totipotent hematopoietic stem cells in responses of the host to tumor bearing.